This invention relates to a transformer for d-c signals in general and more particularly to such a transformer having a primary winding to which the d-c signal is coupled, a core of magnetically soft material and a secondary winding which is acted upon by an electrical variable signal having a polarity which changes each time the core reaches saturation with a current measuring device associated with the secondary of the transformer.
A d-c transformer of this general nature is disclosed in German Auslegeschrift No. 1,153,452. In this known transformer, a square-wave voltage having a polarity which changes each time saturation is reached is applied to the secondary winding. In order to maintain high transformer accuracy, the magnetic material of the core must have a rectangular hysteresis loop. The switching frequency for the square-wave voltage is determined by the speed with which the hysteresis loop is traversed. This switching frequency must be substantially lower than the switching frequency of the semiconductor elements used in switching the voltage. Otherwise, if the switching frequency is too high, the inertia of the semiconductor elements becomes noticeable leading to an inaccurate transmission of d-c signals. The speed with which the hysteresis loop is traversed is a function of the magnetic flux which must be supplied for magnetizing the core. In order that a given speed will not be exceed the flux must not be lower than a given value. However, what this means, in the last analysis, is that the mechanical size of the magnetic core must not be below a given value. As a result, using this known transformer, requirements demanded in computer technology with regard to physical dimensions cannot be met.
In view of these drawbacks, the need for an improved d-c transformer whose magnetic flux and physical dimensions are independent of the switching frequency and which can be built using a core of any magnetically soft material without regard to a particular hysteresis loop is evident.